A comprehensive review of sustainable materials and toolpath optimization in 3D concrete printing

Climate change has emerged as a global challenge due to the substantial carbon emissions and energy consumption. In 2022, the global carbon emissions and energy consumption reached 36.8 gigatons and 14,585 million tonnes of oil equivalent^1,2, respectively. The construction sector is a major contributor to global carbon emission and energy consumption, accounting for 40% and 36% in 2022³, respectively. With the urban population estimated to increase to 68% by 2050, the environmental impacts of the construction sector will continuously increase⁴, underscoring the urgent need for developing sustainable construction technologies.

3D concrete printing (3DCP), also known as additive manufacturing (AM) in the construction sector⁵, offers a promising solution for achieving sustainable construction. 3DCP constructs structures by depositing printable concrete materials layer-atop-layer based on a pre-designed building model. The unique construction process possesses the advantages of enhanced sustainability and design flexibility. For example, a prefabricated bathroom unit (PBU) constructed by 3DCP achieved a reduction of 85.9% and 87.1% in carbon emissions and energy consumption, respectively, compared to that of a mold-cast counterpart⁶.

3DCP has gained much attention from both academia and engineering. Figure 1a shows the rapid growth in the publications and citations related to the keywords of “3DCP” based on data obtained from Web of Science. The number of publications reached 444 and 420 in 2022 and 2023, respectively. In these publications, several review works have been conducted in the fields of 3DCP and its potential applications^7,8,9,10. Wangler et al.⁸ present a technical review of 3DCP from fresh materials to hardened materials and further practical applications. Lu et al.⁹ provide a comprehensive review of the material behaviors of 3DCP. However, the review articles primarily focus on the technical or material advancements of 3DCP, with less attention given to its sustainability aspects. Figure 1b illustrates the growth of publications related to the keywords of “3DCP and Sustainability”. Despite the growing interest in 3DCP, only 46 publications in 2023, approximately 10% of total 3DCP-related publications, focused on sustainability (Fig. 1a). Among these publications, Dey et al.¹¹ provide a comprehensive review of the utilization of industrial wastes in printable materials to improve the sustainability of 3DCP. However, there is a lack of in-depth understanding of how to improve sustainability in 3DCP across its various construction processes.

a Keywords of “3DCP”; b keywords of “3DCP and Sustainability”.

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The typical construction processes of 3DCP include the development of printable materials, structural optimization, toolpath design, and printing¹², as shown in Fig. 2. Each of these processes offers opportunities for enhancing sustainability. At the material level, sustainability can be improved by developing printable materials incorporated with waste materials. The waste materials are used as the substitutions of aggregate and binder contents, thereby reducing the carbon emission associated with the material extraction. At the structural level, the design of hollow structures via topology optimization (TO)^13,14 reduces the material usage and thus enhances sustainability. TO involves the optimization of material distribution to achieve the desired performance. In addition, the design flexibility of 3DCP is compatible with the structural TO. Finally, to implement the optimized structure into 3DCP, toolpath design methods^15,16 are adopted to determine the efficient path for sustainable concrete printing. The integration of sustainable printable materials, TO, and toolpath design techniques with 3DCP represents a promising synergy for future research and sustainability development in the construction sector. However, comprehensive reviews covering these three aspects are currently lacking in the existing literature.

The typical processes include the development of printable materials, structural optimization, toolpath generation, and printing.

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This paper aims to fill the abovementioned research gap by providing a comprehensive review of sustainable materials, structural topology optimization, and toolpath planning for the enhancement of sustainability in 3DCP. Based on the findings of these reviewed articles, the perspectives and methods to enhance sustainability with respect to the abovementioned three aspects of 3DCP are highlighted. Finally, Section 5 conclusions are summarized and future research directions are identified.

Integrating sustainable materials into 3DCP is a potential strategy for enhancing the sustainability of 3DCP¹⁷ since the construction sector increasingly focuses on the recycling of natural resources, reduction in material waste and carbon emissions. The commonly developed 3D printable cementitious materials consist of binder materials (primarily cement), natural fine aggregates, additives, admixtures, and water¹⁸. However, two main challenges impede the development of sustainable 3D printable cementitious materials. Firstly, the high usage of ordinary Portland cement (OPC, 700–800 kg/m³)¹⁸ impacts sustainability due to the associated carbon footprint⁸. Secondly, during the printing process, most developed material mixtures only use fine aggregates for 3DCP due to the limitation of the pumping process and nozzle opening^19,20.

Employing sustainable binder and aggregate alternatives is a potential solution to address these challenges^6,21. This section reviews relevant advancements in adopting recycled aggregates and supplementary cementitious materials (SCMs) into 3D printable materials. Figure 3 shows the number of publications associated with the keywords “3D printed concrete”, “Recycled glass”, “Recycled sand”, “Recycled concrete aggregate”, “Recycled plastics”, “Recycled rubber”, “3D printed concrete”, “Silica fume”, “Rice husk”, “Fly ash”, “Limestone”, “Calcined clay”, “Granulated blast-furnace slag” and “Sustainable” from the Web of Science database. As shown in Fig. 3, a growing academic interest is observed related to recycled aggregates and SCMs. The following sections discuss the performance characteristics and implications of these sustainable materials in 3DCP applications.

The literature study includes research on two main types of sustainable materials, recycled aggregates and SCM, from 2018 to 2023.

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The impact of recycled aggregates, such as recycled glass²², concrete²³, plastics²⁴, and rubber²⁵, alongside SCMs, such as silica fume²⁶, rice husk ash²⁷, fly ash²⁸, limestone²⁹, calcined clay³⁰, and granulated blast-furnace slag (GGBS)³¹, on the fresh and hardened properties of 3D printable materials are analyzed. The analysis underscores the importance of these materials in advancing 3DCP sustainability but also reveals the future potential research direction to mitigate environmental impacts and foster sustainable development in 3D printable cementitious materials^19,32,33.

The primary recycled materials in 3D printed concrete for sustainability enhancement include sand³⁴, glass²², concrete³⁵, plastics²⁴, and rubber²⁵. According to the data from the Hong Kong Environmental Protection Department in 2022³⁶, daily waste generation in Hong Kong includes 222.6 tons of glass and 2336.9 tons of plastics. In addition, concrete and sand, derived mainly from construction and demolition debris and construction waste, account for a considerable portion of the waste, with daily production of construction waste reaching 49,865 tons³⁶. In the blueprint for Hong Kong 2035³⁷, the government proposes a new target concerning “Waste Reduction, Resources Circulation, Zero Landfill”, which presents a significant challenge for the recycling of waste materials in sustainable construction.

In 3DCP, it is essential to achieve a balance of fresh properties and hardened properties for printable materials. Fresh properties such as printability and pumpability, hardened properties such as strength and durability, and sustainability are critical factors for material tailoring^19,38. Recycled aggregates are sustainable alternatives to natural aggregates, helping to conserve natural resources and reduce land waste from landfills^24,39,40,41. This section discusses the various recycled aggregates in 3DCP (see Table 1 for details) to illustrate their impact on material fresh and hardened performance as well as sustainability.

Summarizing the findings from Table 1, the usage of recycled aggregates impacts the fresh properties of cementitious materials. The fresh properties are critical factors, which determine the printability of materials during the printing process. The printability can be characterized by workability, pumpability, extrudability, and buildability³⁴. In the 3D printing process, the most essential steps are conveying mixed materials to the nozzle via a delivery system and depositing materials to build the solid object in a layer-by-layer manner⁴². In the conveying step, the materials are required to have good workability and pumpability, which indicates how easily the material can be conveyed. In addition, extrudability indicates the ability of a material to be extruded with minimal energy consumption during the delivery⁴³. In the deposition step, the materials are required to have good buildability, which indicates how well the materials can be stacked stably.

With respect to workability, research has indicated that the presence of recycled sand, characterized by its high water absorption rate and irregular shape, tends to reduce the workability of concrete⁴⁴. Similarly, incorporating recycled rubber particles with poor shape and rough surfaces has diminished the workability of 3D printed concrete, resulting in the slow relative motion of rubber particles within the concrete mixture, causing reduced processability²⁵. In terms of pumpability, studies conducted by Ting et al.⁴⁵ have shown that adding recycled glass to concrete reduces its pumpability. This phenomenon can be attributed to recycled glass particles’ angular and sharp-edged nature, which obstruct flow and decrease pumpability.

Analyzing the extrudability in recycled aggregate concrete, it has been observed that the high water absorption of recycled sand necessitates the addition of extra water and superplasticizers to enhance the extrudability of 3D printed concrete³⁴. In addition, the water-absorbing nature of surface cracks in recycled rubber can result in reduced extrudability. However, subjecting recycled rubber to heat treatment can partially close these surface cracks, reducing water absorption and significantly improving extrudability⁴⁶.

Finally, with respect to buildability, increasing the substitution rate of recycled concrete aggregates has been found to improve the buildability of 3D printed concrete. Liu et al.’s³⁵ research suggests that the buildability increases with the rising substitution rate of recycled concrete aggregates due to the reduction in concrete density. Conversely, studies involving recycled plastics have revealed that while plastic’s hydrophobic nature enhances material flow, it also delays the hydration reaction of calcium silicate, slowing the thixotropic behavior of concrete and ultimately reducing its buildability^24,41.

In summary, recycled aggregates’ influence on cementation materials’ fresh properties is multifaceted and crucial for 3D printing applications. Workability can be compromised by recycled sand and rubber, while pumpability may be hindered when using recycled glass due to its angular characteristics. Extrudability can be improved with additional water and heat treatment for specific recycled materials. In addition, buildability is positively correlated with higher substitution rates of recycled concrete aggregates, while challenges arise from the delayed hydration reaction of calcium silicate when recycled plastics are involved. These insights underscore the need for careful material selection and processing adjustments to optimize the performance of 3D printable materials.

The mechanical performance of printed structures is paramount for ensuring their structural integrity and safety. Table 1 summarizes the mechanical properties of various types of recycled aggregates, revealing their impact on the mechanical properties of 3D printed concrete. Specifically, incorporating recycled materials such as recycled sand, coarse aggregates, glass, and plastics as sustainable alternatives in concrete leads to decreased compressive strength with increasing substitution rates^22,24,34,35. This reduction in compressive strength can be attributed to the increased porosity within the concrete resulting from the addition of recycled materials, with higher porosity leading to reduced compressive strength^24,39.

Beyond the problem of increased porosity, the bond between recycled aggregates and the cement matrix plays a significant role in the mechanical performance of 3D printed concrete. The smoother surface and sharper edges of recycled glass particles compared to that of natural sand particles may result in weaker bonding between the particles and the cement matrix at the interface transition zone, decreasing mechanical strength⁴⁵. The inherent properties of recycled aggregates also impact the strength of 3D printed concrete. Recycled concrete aggregates containing old mortar and aggregates with adhering old mortar, which have lower mechanical properties, can serve as weak areas of a structure, decreasing mechanical performance³⁵. On the contrary, adding cement-coated modified recycled rubber in 3D printed concrete enhances its compressive strength. This enhancement is primarily attributed to the transformation of the rubber from a hydrophobic material to a hydrophilic material after modification, promoting its interaction with the fresh mortar during mixing and resulting in a more compact interface transition zone within the structure³³.

These findings emphasize the necessity of incorporating recycled aggregates in 3D printed concrete in appropriate amounts after considering the structural integrity and safety to achieve the desired overall properties of 3D printed concrete. As a type of sustainable material, utilizing recycled aggregates in 3DCP can reduce material costs and mitigate environmental impacts⁴⁷. Han et al. indicate that as the proportion of recycled aggregates increases from 0% to 100%, the CO₂ emissions of 3D printed concrete decrease from 5637.647 kg to 5499.505 kg⁴⁸. Cost analyses demonstrate a downward trend in the total cost of 3D printed concrete with the increasing proportion of recycled aggregates. For instance, the costs of 3D printed concrete with recycling proportions of 0%, 50%, and 100% are 12,913.54 CNY, 12,555.77 CNY, and 12,194.97 CNY, respectively⁴⁸. This underscores that increasing the proportion of recycled aggregates can effectively reduce greenhouse gas emissions during concrete production and enhance building materials’ sustainability in the practical application.

This section explores the impact of supplementary cementitious materials (SCMs) on the performance of 3D printable cementitious materials. In the area of 3DCP, a significant aspect is its heavy reliance on OPC compared to traditional concrete¹⁸. Specifically, 3D printable cementitious materials contain more than 20% of OPC, expressed by mass weight due to the requirements of printability¹⁹. Including SCMs in material mixtures is an alternative solution to address his problem. Various types of SCMs have been adopted for the mixture design of 3D printable concrete in the existing literature, such as fly ash, ground granulated blast furnace slag, and calcined clay from various industrial processes⁴⁹. Fly ash, a residue from coal combustion in power plants⁵⁰, and calcined clay, derived from high-temperature treatment of clay materials²⁹, are among these industrially sourced SCM. In addition, GGBS originates from the milling process of waste slag from steel production⁵¹, while silica fume comes from silicon ferroalloy smelting⁵², and rice husk ash is a by-product of rice milling²⁷. Incorporating these SCMs reduces the environmental burden associated with concrete production and addresses the high carbon dioxide emissions from cement production^29,53.

In the selection of SCMs for 3D printable concrete, optimizing characteristics such as fresh properties, mechanical properties, durability, and sustainability is crucial^29,30,54. These attributes directly impact the efficiency of the printing process and the performance of the final structure. Table 2 summarizes the material characteristics of individual SCM used in 3DCP and their impacts on the performance of 3D printable concrete by a systematic literature review.

Based on Table 2, the utilization of SCMs affects the printability of 3D printed concrete. These parameters serve as crucial indicators of the stability and performance of materials during processes such as pumping, extrusion, and bearing continuous printing layer loads. In terms of workability, adding silica fume reduces the workability of 3D printed concrete. This is primarily attributed to the high surface area of silica fume, which easily aggregates with cement particles to form flocculent structures, partially hindering the free flow of water, and therefore, affecting the workability^26,53.

In terms of pumpability and extrudability, the appropriate addition of fly ash and GGBS can enhance the pumpability and extrudability of cementitious materials. This is primarily attributed to the spherical and smooth surface characteristics of fly ash⁵⁵ and GGBS⁵⁶, as shown in Table 2, and therefore, contribute to improving the extrudability of concrete. However, excessive fly ash and GGBS may diminish extrudability due to increased water absorption. As the dosage increases, the water absorption rate rises, resulting in increased viscosity, thereby impeding the extrusion process during 3D printing⁵⁷. The replacement of cement with silica fume²⁶, rice husk ash²⁷, limestone, and calcined clay²⁹ can enhance buildability. For example, torque viscosity rises while flow resistance and thixotropy are decreased with the rise of fly ash-to-cement ratio, negatively impacting the buildability⁵⁵. Conversely, the influence of the silica fume-to-cement ratio shows an opposite trend on rheological properties as compared to that of the fly ash-to-cement ratio. Adding silica fume increases the filler content in concrete, strengthening the interaction between particles and thereby improving the 3D printing performance of the material⁵⁰. Rice husk ash exhibits strong water absorption capability, reducing voids between concrete particles and promoting flocculation and hydration product formation, thereby enhancing the buildability of 3D printed concrete²⁷. The addition of limestone and calcined clay can enhance the buildability due to the reduced water film thickness³⁰.

In summary, incorporating SCMs significantly impacts the workability, pumpability, extrudability, and buildability of 3D printed concrete. While silica fume reduces workability due to its high surface area²⁶, fly ash and GGBS can enhance pumpability and extrudability when added appropriately. However, excessive amounts may hinder extrudability due to increased water absorption⁵⁷. Substituting cement with fly ash, silica fume, rice husk ash, limestone, and calcined clay enhances buildability^26,27,58.

The mechanical performance of 3D printed concrete is crucial for construction practices. Incorporating SCMs can reduce the environmental impact and directly influence the mechanical properties of 3D printed concrete. Studies have shown that materials such as silica fume²⁶, limestone, and calcined clay²⁹ can positively impact the mechanical properties of concrete. Silica fume acts as an inert filler in 3D printed concrete, filling voids, improving pore structure, and therefore, enhancing mechanical performance⁵⁰. Liu et al.²⁶ attributed the improvement in the mechanical properties of silica fume to the fact that silica fume increases the density of the concrete, which increases the pore densities and reduces the number of connecting and oversized pores.

Moreover, the quantity of SCMs added also affects the mechanical properties of 3D printed concrete. Increasing the content of limestone and calcined clay can increase the amount of fine particles in concrete, promoting microstructure development⁵⁴. However, small additions of fly ash and GGBS can enhance mechanical properties but excessive amounts may compromise concrete strength. This is mainly due to that the high amount of replacement of cement with fly ash or GGBS reduces the initial cement hydration at the early stage⁵⁷. As a result, the mechanical performance of 3D printable concrete decreases. Therefore, when designing formulations for 3D printed concrete, it is essential to consider the type, quantity, and interactions of SCMs to achieve optimal mechanical performance and ensure the sustainability and durability of structures.

In 3DCP, the CO₂ emission in the material production stage is 583.1 kg CO₂-eq/m³, 75% of which is contributed by the production of cement and other binder materials¹⁸. Therefore, using SCMs as the substitution of binder materials showed possible advantages in enhancing the environmental sustainability of 3D printable concrete^26,28,54. Most of the reviewed studies focus on the fresh and hardened properties of 3D printable concrete with SCMs, with limited attention to the quantitative carbon emission assessment of the materials. Long et al.⁵⁹ reported that the 3D printable Limestone & Calcined clay cement composites (LC3) reduced carbon emission by 45% and energy consumption by 40%. Conversely, Yao et al.⁶⁰ reported that the carbon emission of printable materials was when geopolymer was used as the binder material. The increased carbon emission of geopolymer was due to the use of silicate (alkaline activator). Liu et al.⁶¹ reported that the printable materials with fly ash showed less carbon emission compared to that of the printable geopolymer concrete. Different conclusions were drawn from the existing articles in terms of the carbon emission of 3D printable materials with SCM. Therefore, to comprehensively assess the sustainability effectiveness of SCMs in 3DCP, additional research is necessary in future works by conducting the quantitative carbon emission assessment.

Traditional design principles and considerations are being re-evaluated to leverage the unique capabilities provided by 3D printing⁶². This section aims to review the specific structural optimization methods and considerations tailored for 3DCP technology, with a particular focus on the potential to create functional, efficient, and sustainable designs using topology optimization approaches.

Structural topology optimization is the process of arranging the distribution of materials within a specified design domain to maximize specific mechanical or physical properties, while adhering to prescribed constraints. This concept arose in 1904 when Michell proposed a theoretical analysis to obtain the lightest truss⁶³. The advent of finite element analysis (FEA) and the development of the widely used homogenization method^64,65 in the late 1980s significantly progressed this concept. Since then, the field has seen substantial advancements, thanks to methods such as Solid Isotropic Microstructure with Penalization (SIMP)⁶⁶, Evolutionary Structural Optimization (ESO)⁶⁷, Bi-directional Evolutionary Structural Optimization (BESO)^68,69, and level set method^70,71. These developments have allowed for more sophisticated and efficient designs and further expanded the possibilities of structural topology optimization. As shown in Table 3, the various topology optimization approaches have continuously evolved to improve their effectiveness and efficiency, which are introduced individually in this section.

After the introduction of the homogenization-based topology optimization method by Bendsoe and Kikuchi⁶⁴ and later developments by Bendsoe⁷², the SIMP method was proposed^73,74. Sigmund⁷⁵ provided a clear explanation of the numerical implementation of the SIMP method in 2001 using a concise 99-line Matlab code. The SIMP method assumes constant material properties for the solid material within the design domain. The design variables in the optimization process are the relative densities of each element, which range between zero and one. The material properties are modeled as the relative material density raised to a power multiplied by the properties of the solid material. During the early 1990s, Xie and Steven initially put forth the Evolutionary Structural Optimization (ESO) method to attain optimal topologies for continuum structures^67,76,77. Subsequently, Querin et al.⁶⁸ and Yang et al.⁷⁸ advanced the ESO method to develop the Bi-directional Evolutionary Structural Optimization (BESO) method. The level set-based topology optimization method utilizes a higher-dimensional embedded function to implicitly represent solid-void interfaces^79,80. In the traditional level set method, the Hamilton-Jacobi equation (PDE) is solved using the velocity normal to the interface^71,81,82. The zero-level contour of the embedded function in the conventional level set method defines the material boundary, serving as the partition between the solid and void domains.

In recent years, a variety of innovative optimization algorithms have emerged to tackle the practical challenges associated with flexible design domains, smooth material boundaries, and complex fabrication constraints. One such method is the Reaction diffusion-based level set (RDLS) approach, which was initially introduced in 2014⁸³. The RDLS method enables the specification of geometrical complexities within the optimal configuration, thereby facilitating the identification of the desired structure shape through the evolution of the level set function. Another notable advancement is the Floating projection topology optimization (FPTO) method, which was unveiled in 2021⁸⁴. FPTO ensures that design variables take discrete values, resulting in more robust and practical optimization outcomes. Lastly, the Node moving-based topology optimization (NMTO) method, introduced in 2023⁸⁵ utilizes a narrowband offset from the structural profile to establish a signed-distance function, which determines the direction of node movement. NMTO aims to optimize the structural topology and enhance its overall performance by manipulating node positions. These cutting-edge methods show great promise for advancing the capabilities of 3DCP and optimizing the production of high-performance structures.

Nowadays, structural topology optimization has become increasingly popular in various fields, including additive manufacturing^69,86, architectural design^87,88, biochemical^89,90, and aerospace engineering^91,92. Among them, the high design flexibility of 3DCP makes it compatible with topology optimization to decrease material usage and improve sustainability. With the integration of these approaches and 3DCP, it becomes possible to create intricate designs that are both structurally sound and resource-efficient.

To find an appropriate method for 3DCP, the benefits and limitations of each topology optimization method should be fully understood, which are introduced and summarized in this subsection. The key scientific differences between the various topology optimization methods include mathematical formulation, optimization algorithms, material models, sensitivity analysis, and post-processing techniques.

The advantages and disadvantages of these topology optimization methods can be concluded to judge whether they can be integrated with 3DCP to fabricate efficient and environmentally friendly structures. For instance, the homogenization method allows for accurate computation of material properties using a systematic approach to obtain optimal topology. However, it may not be suitable for structures with complex material distributions and may struggle with handling geometric complexities. The SIMP method is advantageous as it provides a simple and effective way to model material properties and incorporate manufacturing constraints. Nevertheless, it produces designs with intermediate densities and may suffer from numerical instabilities. Next, the ESO method offers improved utilization of material resources by gradually removing ineffective material but may require a large number of iterations and struggle with complex geometries. Similarly, the BESO method efficiently optimizes structures by employing fundamental strategies but may produce designs with checkerboard patterns and require careful parameter tuning. On the other hand, the conventional level set method utilizes higher-dimensional embedded functions to implicitly represent solid-void interfaces accurately, which can handle topological changes during the optimization process. Nonetheless, it requires careful handling of interface tracking to avoid spurious geometries and may suffer from numerical diffusion and grid-related issues.

On the other hand, the RDLS method allows for specifying geometrical complexity but requires significant computational resources. Besides, this method is sensitive to parameter settings. The FPTO method incorporates floating projection constraints and heuristically simulates 0/1 constraints of design variables, leading to discrete and practical solutions, that provide robust optimization results by considering upper and lower bounds. However, the method’s heuristic nature may not guarantee global optimality, and it may require careful tuning of parameters to balance feasibility and optimality. The NMTO method establishes a signed-distance function to determine node-moving directions, allowing for efficient topology optimization, complex structure design, and flexibility in node manipulation. The disadvantage of the NMTO method is that it may struggle with handling complex boundary conditions and geometric constraints. These are just some general advantages and disadvantages of the topology optimization methods mentioned.

In summary, the suitability of each method regarding 3DCP depends on specific applications and requirements. Different topology optimization methods employ various mathematical formulations to represent and solve the optimization problem. Each formulation has its advantages and limitations in terms of modeling flexibility, convergence behavior, and computational efficiency. Besides, topology optimization methods may differ in the sensitivity analysis approach employed to evaluate the influence of design changes on the objective function and constraints. After obtaining an optimized design, different methods employ various post-processing techniques to interpret and convert the obtained results into manufacturable forms. These techniques can include filtering, mesh smoothing, or shape reconstruction algorithms. The selection of post-processing techniques impacts the final quality, manufacturability, and practicality of the optimized design.

Structural topology optimization has been widely applied in the field of 3DCP, due to the benefits to create efficient and optimized structures. By combining these two techniques, engineers can maximize the use of material, reduce weight, and enhance load-bearing capabilities, resulting in more sustainable and cost-effective structures.

The emergence of 3DCP technology has revolutionized the field of structural design by providing unprecedented freedom in creating intricate geometries and customized structures⁹³. This capability opens up new opportunities for designers to push the boundaries of traditional design principles^94,95. By harnessing the inherent freedom of design, 3DCP can create structures that are aesthetically appealing and optimized for performance and functionality⁸⁷. For instance, the optimization of material distribution in 3DCP is a vital research direction to minimize material waste and optimize structural efficiency^14,96. Since the last decade, structural topology optimization has been increasingly applied in 3DCP^97,98. Figure 4 shows the research article number in the last decade integrating different topology optimization approaches and 3DCP using the keywords “3D printed concrete”, “Homogenization method”, “SIMP method”, “ESO method”, “BESO method”, “Level set method”, and “Phase field method” based on data obtained from the Web of Science database. This section focuses on the approaches that have been explored to achieve structural topology optimization in 3DCP. These include using additive manufacturing techniques to build complex geometries and incorporating reinforcement elements during the printing process^14,86,99. Existing works^96,97 have demonstrated the ability to optimize the internal structure of concrete components, resulting in improved mechanical properties and enhanced performance.

The literature study includes research on the application of six typical optimization methods from 2014 to 2024.

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The integration of topology optimization and 3DCP has the potential to enhance the performance and resource efficiency of buildings. With the increasing emphasis on sustainable and eco-friendly practices, optimized structural design has emerged as a critical strategy to reduce material usage while maintaining structural strength^99,100. For instance, the varying physical properties present in functionally graded materials can be customized to meet specific requirements, all while making efficient use of material resources¹⁰¹. Building on the multi-material BESO method, a novel approach to 3DCP structural design was introduced¹⁰². In this approach, 3DPC components primarily experience compression without the need for extra reinforcement. Instead, they synergistically collaborate with tensioned steel cables to create an effective composite structural system. The previous study⁹⁶ examined the production process of a topology-optimized 3D printed concrete bridge structure, highlighting its significant deviation from the manufacturing procedures of conventional concrete structures. Yang et al.¹⁰³ presented an integrated design method for 3DCP by incorporating extrusion-based manufacturing characteristics into the topology optimization algorithm. Lightweight structures tend to have better seismic performance, increased durability, and reduced energy consumption compared to their heavier counterparts⁶¹. In addition, lighter structures require less foundation support, resulting in cost savings during construction¹⁰⁴. Since construction activities are responsible for a significant amount of carbon emissions, reducing the amount of material used can significantly decrease the carbon footprint of a building.

Several examples of a combination of topology optimization and waste materials have been achieved using additive manufacturing^105,106. These technologies provide benefits including minimized waste materials, accelerated construction timelines, and the capacity to create distinctive designs with intricate details. In addition, they classify large-scale 3DCP technologies, emphasizing the importance of optimizing printing ink to enhance economic and environmental results by utilizing waste materials in 3DCP applications. The combination of topology optimization and waste materials offers numerous benefits. Firstly, it promotes sustainable design practices by utilizing recycled or waste materials, contributing to the circular economy and reducing waste. Secondly, it helps reduce costs as waste materials are often less expensive or even available for free compared to conventional materials. In addition, incorporating waste materials into the design improves resource efficiency by minimizing the need for extracting and processing new materials. Moreover, the unique properties of waste materials can enhance the performance of the optimized design, such as strength, durability, or lightweight. This combination also encourages innovation and creativity by exploring unconventional design solutions.

In summary, the integration of topology optimization and 3DCP can enhance the performance and resource efficiency of buildings. The impact of structural lightweighting on seismic performance, durability, and energy consumption makes it a compulsory consideration in achieving resource efficiency. In terms of future research directions, further advancements in structural topology optimization for 3DCP are anticipated. This includes developing advanced algorithms that can handle anisotropic, large-scale optimization problems and integrating multi-material printing capabilities. In addition, research efforts could focus on exploring the potential of bio-inspired design principles and incorporating functional requirements such as interlocking, thermal insulation, and acoustic performance into the optimization process.

Toolpath design is a critical aspect of 3DCP as it directly impacts the quality, efficiency, and structural integrity of the printed components. Firstly, toolpath design takes into account material-related problems, such as the flowability and workability of the concrete mixture. By carefully planning the toolpath, engineers can ensure that the material is properly deposited, minimizing problems, such as clogging or inconsistent layering. Toolpath design also addresses process-related concerns, such as the prevention of sagging or deformation during printing. Optimizing the toolpath by the integration of factors such as load-bearing capabilities, stress distribution, and reinforcement placement, can enhance the structural integrity of the printed components.

Toolpath planning determines the success of the 3DCP process. Toolpath design involves mapping out the trajectory and deposition strategy of the printing toolhead to ensure accurate material placement and optimal structural integrity^101,107,108. By carefully coordinating the movement of the toolhead, designers can achieve precise layering, intricate geometries, improved sustainability, and desired material properties in the printed structure. Xia et al.¹⁰⁹ proposed an integrated design method to improve the mechanical performance and manufacturability of material extrusion structures according to the technical characteristics of material extrusion. The technical aspects of toolpath planning encompass various considerations, such as path optimization^110,111,112, layer sequencing^113,114, manufacturing constraints^14,115,116, and support structure generation^86,117,118.

Figure 5 illustrates the number of publications during the past decade related to the keywords “3D printed concrete”, “Extrusion-based toolpath design”, “Geometric toolpath design”, “Toolpath visualization”, “Manufacturing constraints”, “Topology optimization-based toolpath design”, “Sliced toolpath design”, and “Toolpath design efficiency/performance” based on data obtained from the Web of Science database. Path optimization algorithms aim to minimize print time, reduce material waste, and enhance printing efficiency by optimizing the toolhead’s movement trajectory. Layer sequencing determines the order in which layers are printed to ensure stability and prevent collapse during the printing process. Material flow control involves adjusting the printing parameters, such as nozzle speed and extrusion rate, to achieve consistent material deposition and avoid defects. Lastly, support structure generation ensures the stability of overhanging or complex geometries during printing.

The literature study includes research on different toolpath design approaches from 2016 to 2024.

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In recent years, there have been key research developments^14,15,96,119 in toolpath design and optimization. One of the key areas of focus has been on optimizing toolpaths for material efficiency and print time reduction. Researchers have explored various toolpath design methods with the instruction of topology optimization to achieve efficient and environmentally friendly structures. In addition, advancements in path optimization algorithms^110,111,112, layer sequencing^113,114, and support structure generation^86,117,118 have helped to enhance the printing efficiency and accuracy of 3DCP. Two novel printing techniques, “knitting” and “tilting” filaments, were proposed to address the anisotropy inherent in 3D printed ECC, emulating the natural crossed-lamellar structure of conch shells¹²⁰. Three-dimensional spatial paths were devised to distribute tensile and flexural resistance in multiple directions and establish an interwoven interface system to enhance the strength of the structure.

Toolpath planning includes the strategic arrangement of the printing toolhead’s movement paths and deposition patterns to achieve the desired structural form^{121,122,123,124}. This section aims to highlight the significance of toolpath planning in 3DCP and topology optimization. Existing methods for toolpath design in 3DCP involve a combination of computational algorithms, simulation techniques, and empirical knowledge. These methods consider various constraints and challenges, including printer limitations¹⁴, geometric complexity¹⁶, surface finish requirements¹²⁵, overhang (self-support) problem⁸⁶, interlocking¹²⁶, and stability¹²⁷ during the printing process. They aim to generate toolpaths that maximize printing efficiency while ensuring the structural integrity and quality of the final product.

The toolpath design methods displayed in Fig. 5 can be integrated with 3DCP to fabricate efficient and high-performance structures depending on the fabrication requirements. Extrusion-based toolpath design in 3D concrete printing refers to the process of planning and creating the specific paths along which the extrusion nozzle will move to deposit layers of concrete material in a three-dimensional printed structure. Extrusion-based toolpath design^128,129 offers several advantages. It allows for the generation of toolpaths tailored to the specific material deposition process, resulting in efficient and optimized printing trajectories. By considering the extrusion process, this method can minimize print time, reduce material waste, and enhance printing efficiency. However, it may be limited in its ability to handle complex geometries and struggle with intricate support structure generation. Geometric toolpath design^16,130 focuses on creating toolpaths based on the geometric characteristics of the part being printed. This approach can lead to precise toolpaths that align with the part’s geometry, potentially reducing material waste. However, it may be less effective in optimizing toolpaths for overall printing efficiency and may struggle with handling complex layer sequencing. Toolpath visualization^131,132 provides a visual representation of the toolpaths, aiding in the identification of potential issues such as collisions, inefficient trajectories, or inadequate support structures. While it can help in identifying and addressing these issues, it may not actively optimize the toolpaths for print time, material waste, or printing efficiency. This method allows for precise control over layer sequencing and material flow control, ensuring stable and accurate printing. However, it may require additional computational resources and not fully optimize toolpaths for overall printing efficiency.

Toolpath design can be integrated with topology optimization to generate better performance^103,133. Topology optimization-based toolpath design integrates the principles of topology optimization into the generation of toolpaths. By considering material deposition constraints and printing process dynamics, this method aims to create toolpaths that are not only geometrically optimized but also aligned with manufacturing constraints and support structure requirements. This approach can lead to highly efficient toolpaths that minimize print time, material waste, and enhance overall printing efficiency.

In summary, each of these toolpath design methods offers unique advantages and considerations. The selection of the most suitable method depends on the specific printing requirements, material characteristics, geometric complexity, and manufacturing constraints of the part being printed.

The impact of toolpath optimization on the quality and efficiency of 3DCP has garnered significant attention. This section aims to analyze how optimized toolpaths positively influence printing quality and efficiency, emphasizing the reduction of waste and energy consumption. Advanced algorithms and computational models^119,127,134 are being developed to strategically plan the movement paths and deposition patterns of the printing toolhead, enabling precise material placement and optimized structural performance. A well-planned toolpath can result in structurally sound and aesthetically pleasing printed structures, while inadequate planning can lead to issues like material sagging, poor bonding between layers, or excessive material use^15,135,136. Therefore, understanding and optimizing the toolpath planning process is vital for successful and reliable 3DCP^137,138. Furthermore, toolpath planning also provides opportunities for customization and innovation in construction^16,125. With the ability to precisely control the deposition pattern and material properties, designers can explore novel architectural forms, integrate functional features, and optimize performance characteristics.

Through systematic toolpath planning, it becomes possible to mitigate issues such as over-extrusion, uneven material distribution, and inaccuracies in layer deposition, ultimately leading to superior printing quality^112,139. Moreover, the relationship between toolpath planning and material efficiency is paramount in the context of sustainable manufacturing practices. Optimized toolpaths contribute to the reduction of material waste and energy consumption by streamlining the printing process. Efficient toolpaths enable precise material deposition, minimize unnecessary movements, and optimize the use of support structures, thereby reducing material consumption and enhancing overall sustainability in 3DCP^132,140,141.

The technical considerations involved in toolpath optimization for 3DCP encompass path optimization algorithms, print speed adjustments, and support structure generation. Path optimization algorithms aim to minimize print time and reduce material waste by optimizing the toolhead’s movement trajectory, while print speed adjustments ensure consistent material flow and deposition¹³². In addition, support structure generation and layer sequencing contribute to the stability and efficiency of the printing process⁸⁶. Real-world case studies provide valuable insights into the benefits and challenges associated with toolpath optimization in construction projects^142,143.

In terms of future research directions, there is a requirement to address additional constraints for the practical usage of 3DCP. For instance, the development of artificial intelligence empowered toolpath design methods for structures with complex geometric features. The integration of real-time monitoring and feedback systems into the toolpath design process can help improve accuracy and adaptability during printing. In addition, considering sustainability aspects, such as the use of recycled materials or minimizing waste, presents another avenue for future research in toolpath design for 3DCP.

This study presents a comprehensive overview of three vital aspects integrated with 3D concrete printing (3DCP) that contribute to enhancing sustainability in the construction sector. The first area of focus is sustainable material, which involves optimizing the constituents of printable materials through the recycling of waste materials into aggregates and supplementary cementitious materials. This approach reduces the environmental impact of the materials but also enhances the economic viability of 3DCP. The second vital area discussed is structural optimization, which plays a crucial role in maximizing structural performance and efficiency by rearranging material distribution. This optimization leads to improved structural integrity, reduced material usage, and minimized construction time and cost. Lastly, advances in toolpath planning have significantly improved the quality and efficiency of 3DCP. By strategically planning the movement paths and deposition patterns of the printing toolhead, toolpath optimization enhances printing accuracy, minimizes defects, and improves overall structural integrity. Furthermore, the review article also explores the influence of printing parameters on the quality and integrity of printed structures, providing valuable insights for future research and development in the field. By investigating the synergies between these three elements, this research aims to provide valuable insights for advancing sustainable and efficient building practices through the implementation of 3DCP technology.

The future of 3DCP in the construction sector is promising, while more systematic works are required to facilitate the practical application and sustainability of 3DCP:

• Integration of Advanced Technologies: Future research should focus on integrating advanced technologies such as artificial intelligence and robotic control into toolpath optimization. These technologies can be adopted in the material design, system integration, and real-time optimization of printing processes.

• Development of New Algorithms: There is a need for the development of new algorithms for toolpath optimization that can address specific challenges in 3DCP, such as handling complex geometries, optimizing material flow, and managing overhangs. These algorithms should also aim to optimize multiple objectives simultaneously.

• Exploration of Novel Applications: Future research should explore novel applications of toolpath optimization in construction, such as printing complex architectural forms, integrating functional features, and creating customized structures. The potential of toolpath optimization in challenging environments, such as underwater or in space, should also be investigated.

Systematic literature review

This review article employs a systematic literature review approach based on established practices in additive manufacturing for construction to explore the intersections between 3DCP, material sustainability, structural topology optimization, and toolpath design. The Web of Science Core Collection, including indices such as SCI, SSCI, SCI-Expanded, and ESCI, is utilized to gather diverse publications until December 2023, encompassing journal articles, conference proceedings, books, and reports. A three-stage review method is meticulously designed to ensure objectivity and reproducibility.

Initially, relevant keywords, including “3D concrete printing,” “sustainable material,” “structural topology optimization,” and “toolpath design,” are defined to ensure a focused review. The literature reviews for sustainable material, TO, and toolpath design sections are conducted independently by different researchers. In the first stage, 1033 papers related to 3DCP are identified, with further breakdowns of 400 papers for sustainable material, 472 for structural topology optimization, and 161 for toolpath design. In the second stage, manual screening is conducted based on predefined criteria, including methodology robustness, published year, bibliographic information, and sustainability considerations. Comparative analysis results in the identification of 476 papers, comprising 245 for sustainable material, 136 for structural topology optimization, and 95 for toolpath design, as displayed in Figs. 3, 4, and 5. In the third stage, the literature was further narrowed down to 160 references for inclusion in this review according to the specific criteria, including published journals, impact in the field, and number of citations. This three-step screening procedure guarantees that the literature review remains focused and relevant.

An analytical synthesis is then performed to summarize the primary studies of additive manufacturing in construction. The 160 studies obtained by the screening procedure are integrated systematically and classified into three sections according to their context, study design, and outcomes. The references cited in the sections on sustainable material, structural topology optimization, and toolpath design are 61, 76, and 23, respectively. In conclusion, the systematic literature review methodology minimizes reliance on subjective judgments, mitigates personal biases and errors, and upholds the integrity of scholarly research¹⁴⁴.